Screw pumps are rotary, positive displacement pumps that use two or more screws to transfer high or low viscosity fluids or fluid mixtures along an axis. Generally, a three-screw pump is a positive rotary pump in which a central one of three screws is motor-driven, and the two further screws are idlers meshing with diametrically opposed portions of the driven central screw, the idlers acting as sealing elements that are rotated hydraulically by the fluid being pumped. The volumes or cavities between the intermeshing screws and a liner or casing transport a specific volume of fluid in an axial direction around threads of the screws. As the screws rotate the fluid volumes are transported from an inlet to an outlet of the pump. In some applications, these pumps are used to aid in the extraction of oil from on-shore and sub-sea wells.
Often the liquids pumped through these pumps include entrained solids, such as sand. The presence of sand and other solids can cause damage to the pump internals, most notably to the pump casing, where the solids can pass between the screws and the casing. Substantial wear of the pump casing can undesirably result in reduced discharge flow rates. Repair of pump casings can be expensive, and thus, many manufacturers line the pump casing with a self-repairing liner material. Such liners are typically made from material that is much softer than the casing and screws. Thus, damage due to entrained solids is borne by the liner and not the more expensive casing. Such liners may be “self-repairing,” in that over time, scratches and gouges caused by contact with entrained solids may be smoothed over, mitigating their impact on performance of the pump.
While such liners can improve pump lifecycle, periodic liner refurbishment is still required. A difficulty remains, however, in determining when liner replacement should occur. As noted, liner degradation may manifest itself in reduced output flow from the pump. Where multiple pumps serve a single outlet, however, it can be difficult to identify which pump may be the cause of reduced overall flow. Thus, it would be desirable to provide a system and method for continuously monitoring wear of pump casing liners so that repair can be performed in a timely manner.
Wear monitoring systems, in general, are known. For example, U.S. Pat. No. 6,945,098 to Olson discloses a wear detection system for use in determining wall thinning in hydrocyclone applications, U.S. Pat. No. 6,290,027 to Matsuzaki, U.S. Pat. No. 5,833,033 to Takanashi, and U.S. Pat. No. 4,274,511 to Moriya disclose systems for detecting wear of brake pads, and U.S. Pat. No. 3,102,759 to Stewart discloses a system for detecting wear of journal bearings. The problem with these systems is that they may not be as accurate as desired. This is because the systems employ wear sensors made of materials that have compositions and properties different from the compositions and properties of the components being monitored. Owing to such differences, the sensors may wear at a faster or slower rate than the monitored components. As will be appreciated, where sensor wear is not consistent with component wear, the accuracy of the monitoring system is adversely affected.
Thus, there remains a need for an improved wear monitoring system that can continuously monitor wear of pump casing liners so that repair can be effected in a timely manner. Such a system should overcome the deficiencies inherent in current systems, and should be highly accurate. It would also be desirable to provide a system and method for storing liner wear information so that wear trending can be accomplished.